The moon
formed after a nasty planetary collision with young Earth, yet it looks odd next
to its watery orbital neighbor. Turns out it really is odd: Only about one in
every 10 to 20 solar systems may harbor a similar moon.
New
observations made by NASA's Spitzer
Space Telescope of stellar dust clouds suggest that moons like Earth's
are—at most—in only 5 to 10 percent of planetary systems.
"When
a moon forms from a violent collision, dust should be blasted everywhere,"
said Nadya Gorlova, an astronomer at the University of Florida in Gainesville who analyzed the telescope data in a new study. "If there were lots of
moons forming, we would have seen dust around lots of stars. But we
didn't."
Gorlova and
her team detail their findings in today's issue of the Astrophysical Journal.
Violent
birth
Shortly
after the sun formed about 4.5 billion years ago, scientists think a vagrant
planet as big as Mars smacked into infant Earth and ripped off a chunk of our
home's smoldering mantle. The rocky, dusty leftovers fell into orbit around our
wounded planet, eventually coalescing into the moon we see today.
The
scenario is unique among other moons in the solar system, which formed side-by-side
with their planet or were captured by its gravity. Gorlova and her colleagues
looked for the dusty signs of similar smash-ups around 400 stars, all about 30
million years old—roughly the age of our sun when Earth's moon formed.
Only one of
all the stars they studied, however, displayed the telltale dust. Considering the
frequency of planetary solar systems, the amount of time the dust should stick
around and the window for moon-forming collisions to occur, the scientists were
able to peg the frequency of extrasolar bodies that formed like our moon.
The
estimate, however, is possibly a generous one.
"We
don't know that the collision we witnessed around the one star is definitely
going to produce a moon," said study co-author George Rieke, an astronomer
at the University of Arizona in Tucson, "so moon-forming events could be
much less frequent than our calculation suggests."
Odd moon
out?
Planetary
scientists like Gorlova and Rieke think infant solar systems can form moons
between 10 and 50 million years after a star forms. That only a single star with
collision-generated dust could be found in their latest research, the
astronomers said, indicates that the 30 million-year-old stars in the study
have finished making their planets.
"Astronomers
have observed young stars with dust swirling around them for more than 20 years
now," said Gorlova, noting that the dust could be collision-derived or primitive
planet-forming material. "The star we have found is older, at the same
age our sun was when it had finished making planets and the Earth-moon system
had just formed in a collision."
While most the
our type of moon may be rare, astronomers think there are billions of rocky
planets out there with plenty
of moons orbiting around them. The upshot for lunar lovers? There could be
millions—or billions—of Earth-like moons drifting through the cosmos.
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